@ilia might give a more complete answer, and overall I agree with your point, especially that STRK will be used at full potential with the decentralized network setup.
But I think using STRK as gas token even before enables a few things, that are worth contemplating:
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Giving users things to use their tokens for: STRK will certainly be available to use in different protocols, but letting users use it for gas fees is useful.
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Empowering the foundation to start no-gas onboarding: It’s a secret to no one, the foundation has tokens to allocate for subsidizing gas fees, if STRK is enabled as gas fees the foundation could start sponsoring part of the gas fees to drive more adoption, for onboarding or using dapps.
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Starting to decentralize the governance through devonomics: The foundation has announced the devonomics, rewarding dev teams with ETH. If the sequencer collects STRK, it can enable the foundation to distribute and decentralize the network across core devs and app builders in an efficient way, using the devconomics framework.
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Get an idea about the appetite for users to pay in STRK rather than ETH: The network will fully quote the gas in STRK as it decentralizes, it would be good to have an initial idea about the appetite of users to pay in STRK while they still can pay in ETH. So that we’ll be able to know if we need paymasters before decentralization and what to prioritize in terms of UX/decentralization tradeoff.
So we get all of this, and again just to make things clear, the only complicated consequence introduced by the STRK payment is possible bad debt for the sequencer operator (no user fund touched), capped by a maximum of (gas fees required to settle one L2 block on L1) * (number of blocks on Starknet misquoted) or potential too high STRK gas price, but users can still pay in ETH in that case.